Propane is a gas, a derivative of natural gas and petroleum. It is one of the many fossil fuels that are included in the liquefied petroleum (LP) gas family. Because propane is the type of LP-gas most commonly used in the United States, propane and LP-gas are often used synonymously.
Under normal atmospheric pressure and temperature, propane is a gas. Under moderate pressure and/or lower temperatures, however, propane changes into a liquid. Because Propane takes up much less space in its liquid form, it is easily stored as a liquid in pressurized tanks. When propane vapor (gas) is drawn from a tank, some of the liquid in the tank instantly vaporizes to replace the vapor that was removed.
Homes and businesses use about one-third of the propane consumed in the U.S. Propane is used mostly in homes in rural areas that do not have natural gas service. More than 20 million households use propane to meet some of their energy needs, while 16 million households use propane as their main heating source. Homes that use propane as a main energy source usually have a large propane tank outside of the house that stores propane under pressure as a liquid.
Because home space heating is a primary use of propane, demand is much higher during the winter months. Residential users of propane typically have a 250-500 gallon tank installed by a local propane dealer and accessible to delivery trucks for refilling. Depending on the climate, a typical residential tank is filled three to four times per year. A residential tank is usually owned by the propane dealer and rented to the residential customer for an annual fee.
Propane dealers typically operate out of bulk storage plants that include one to two 30,000 gallon storage tanks. A single dealer will usually be able to effectively service a 35 mile radius around the plant, though in less populated regions a much larger service area may be necessary to achieve sufficient volume. Propane is delivered to customers by bulk delivery trucks or “bobtails” which typically hold from 1,800 to 3,000 gallons of propane. Customer tanks usually make up the largest portion of a dealer's assets.
Obviously, different size tanks and different usage rates for customers over a large area can make it very challenging for a dealer to keep all of his customers' tanks filled. The quantity of liquid propane stored and remaining on customer propane tanks needs to be measured frequently so that the propane dealer can manage his own inventory of bulk propane, efficiently schedule deliveries, and most importantly keep his customers supplied with propane. There are also significant safety concerns associated with propane tank levels since empty or overfilled tanks can be very dangerous. Further, costs associated with delivery, including wages for delivery personnel and vehicle operation and fuel costs, are a significant portion of a dealer's operating expenses. For this reason, dealers must try to maximize the ratio of gallons of delivered propane per mile traveled by delivery vehicles in order to lower delivery costs.
Traditionally, the standard practice was for propane dealers to periodically visit each tank and visually read a gauge located on the tank in order to determine whether the tank needed refilling. If the tank level was low, it would be refilled; if not, the delivery truck had essentially wasted a trip. As could be expected, this highly inefficient practice contributed to higher costs, both for the dealer and the customers.
For this reason, a number of forecasting methods were developed to give dealers a better idea of how much propane a customer was using and when more should be delivered. Since propane is primarily used as a heating fuel, the typical forecasting method involved factoring temperature and historic customer usage rates. A Degree Day is a unit used to measure how cold it has been over a 24-hour period. The base temperature for Degree-day calculations is 65 degrees. The actual temperature is compared to the 65° base temperature and if the temperature is lower, the difference is the number of Degree-days for that day. For example, if the average temperature for a 24-hour period was 60° , that would be 5° less than the base temperature of 65° , so we would have 5 Degree-days for that 24-hour period. Another concept, referred to as the K-factor, is used to get an idea of the propane usage rate for a customer. The customer's K-factor is the number of Degree-days that it takes for a given customer (or burner(s) associated with a given tank) to use one gallon of propane.
From these two measurements, a dealer could get a better idea as to when more propane should be delivered. For example, a customer with a 275-gallon propane tank with a historic K-factor of 5 could be expected to go 1375 Degree-days before the tank is empty. However, since an empty tank is a dangerous condition (plus it means the customer is out of fuel) delivery will need to be made before the 1375 Degree-days have elapsed. Further, these types of forecasting methods cannot account for unexpected periods of higher or lower than normal propane usage. Since this kind of forecasting is merely an estimate, a substantial margin of error must be built into the delivery schedule. This results in more deliveries of lower amounts of propane and consequently higher dealer delivery costs.
For many years, various optimal vehicle routing computer programs have been available to minimize the mileage and travel time associated with making desired deliveries using vehicles with known capacities. All such methods in the prior art, however, necessarily depend upon various methods of forecasting a customer's propane usage since the last delivery and, as discussed above, such forecasting methods are never completely reliable.
More recently, remote monitoring systems have been used to allow remote transmission of data relating to the level of the liquid gas contained in customer tanks. This allows for the delivery of fuel or other fluids to the storage tank on an “as-needed” basis. Such monitoring systems are typically more accurate than forecasting systems and increase the efficiencies of the propane supplier.
Storage tank monitoring systems currently in use typically include a float sensor within a storage tank that measures the level of fluid and the temperature within the storage tank. For remote monitoring systems, data from the sensor is transmitted through some type of communication network to a data processing unit or display device. Typically, the data processing unit is a computer that decodes and stores the data using specialized software. The information received by the data processing unit provides for the monitoring of each specific storage tank individually.
One remote monitoring system known in the prior art makes use of RF broadcasting to communicate data from the sensor to the data processing unit. Such systems are relatively inexpensive, however, they have very limited range. The data processing unit would typically be mounted in a delivery truck which would have to be in the vicinity of the customer's tank for the level to be reported.
Another prior art system uses a modem and ordinary telephone lines to communicate data from the sensor to the data processing unit. Typically, such a system will use the modem to call in and signal the data processing unit when the propane in a tank reaches a pre-determined level. The customer's phone line must be free for the system to work.
Other prior art systems used to monitor liquid volume in tanks make use of satellite or cellular communications. However, each of these systems also suffers from disadvantages in certain circumstances. For example, many satellite systems require an externally mounted satellite dish with the proper exposure. Additionally, two-way communication requires expensive equipment and installation. Cellular systems are not practical in certain locations due to a lack of cellular coverage.
No matter which communication scheme is used, the data received from the sensor is often confusing and can require significant time to decode and format into a useful form. Even then, it is still difficult for a dealer to interpret the data or use the information to optimally organize his trucks and routes. Further, a dealer must be able to access the data processing unit in order to make use of the data, and this typically requires that the dealer be physically in his office in order to monitor his business. Also, certain tank conditions, such as an over-fill or gas leak, require immediate attention. It is sometime difficult to identify certain dangerous conditions. For example, data showing rapidly falling fuel levels could indicate a leak or could simply result from a period of high fuel usage. For events occurring outside ordinary business hours, either the dealer must have an employee monitoring the system 24 hours a day or else these events will not be corrected until the next business day.
Propane dealers also face economic challenges arising from the seasonal nature of propane demand. As discussed above, demand for propane is high during the winter months, but much lower during summer. The propane dealer has a significant investment in tanks, trucks, employees, and infrastructure, and yet he receives a poor return on this investment during periods of low demand.
What is needed is an improved system for monitoring the levels of propane or other consumable liquid in remotely located storage tanks and to better coordinate delivery of liquid to those tanks.